The present invention relates to methods for removing ceramic coatings. More particularly, this invention is directed to a method for removing a layer of a zirconia-containing ceramic layer, such as a thermal barrier coating of yttria-stabilized zirconia (YSZ), from a surface, such as that of tooling of a deposition apparatus or that of a component of a gas turbine engine.
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles.
Coating systems capable of satisfying the above requirements typically include a metallic bond coat that adheres the TBC to the component. Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant overcoat alloy such as MCrAlY (where M is iron, cobalt and/or nickel). Metal oxides, such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, have been widely employed as TBC materials. TBC is typically deposited by flame spraying, air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique such as electron beam physical vapor deposition (EBPVD), which yields a strain-tolerant columnar grain structure. These coating techniques require tooling to position, rotate and mask components being coated, such that the coating process can be controlled to shield or coat selected portions of the components.
Though significant advances have been made with coating materials and processes for producing both the environmentally-resistant bond coat and the TBC, there is the inevitable requirement to remove and replace the TBC under certain circumstances. For example, removal may be necessitated by erosion or impact damage to the TBC during engine operation, or by a requirement to repair certain features such as the tip length of a turbine blade. Removal of the TBC may also be necessitated during component manufacturing to address such problems as defects in the coating, handling damage and the need to repeat noncoating-related manufacturing operations which require removal of the ceramic, e.g., electrical-discharge machining (EDM) operations. Because the equipment, tooling and maskants employed in the deposition of TBC tend to become coated with the TBC material, a need also arises to periodically remove TBC from these components in order to ensure their proper function and operation. As an example, unwanted TBC must often be removed from maskants after only a few coating cycles.
Current state-of-the-art methods for repairing components protected by TBC often result in removal of the entire TBC system, i.e., both the ceramic TBC and the bond coat, after which the bond coat and TBC must be redeposited. One such method is to use abrasives in procedures such as grit blasting, vapor honing and glass bead peening, each of which is a slow, labor-intensive process that erodes the TBC and bond coat, as well as the substrate surface beneath the bond coat. With repetitive use, these procedures eventually destroy the component by reducing the wall thickness of the component. This disadvantage is particular acute with diffusion aluminide bond coats, which have a diffusion zone that extends into the substrate surface of the component. Damage to diffusion aluminide bond coats generally occurs by the fracturing of brittle phases in the diffusion zone, such as PtAl2 phases of a platinum-aluminide bond coat, or in the additive layer, which is the outermost bond coat layer containing an environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material. Damage is particularly likely when treating an air-cooled component, such as a turbine blade or vane whose airfoil surfaces include cooling holes from which cooling air is discharged to cool its external surfaces.
With respect to the removal of TBC from tooling, the requirement that TBC be repeatedly removed to maintain the operability of the tooling drastically shortens the life of the tooling, leading to frequent tooling replacement. Because of the high temperature processes used to deposit TBC, tooling is often formed of superalloy materials, such as Hastelloy X, with the result that tooling replacement costs can be high.
In view of the above, significant effort has been directed to developing nonabrasive processes for removing TBC. One such method is an autoclaving process in which the TBC is subjected to elevated temperatures and pressures in the presence of a caustic compound. This process has been found to sufficiently weaken the chemical bond between the TBC and bond coat layers to permit removal of the TBC while leaving the bond coat intact. However, suitable autoclaving equipment is expensive, and autoclaving techniques have been incapable of removing ceramic from the cooling holes of air-cooled turbine blades and vanes. Consequently, cooling holes are likely to become constricted when new TBC is deposited, which is detrimental to the performance of the component. Other known techniques for removing TBC from coater tooling and gas turbine engine components include fluoride ion cleaning and high temperature treatments with chloride. However, each of these techniques generally has the disadvantage of being slow, which significantly limits throughput and results in relatively long turnaround times.
A more rapid technique for removing TBC is disclosed in U.S. Pat. No. 5,614,054 to Reeves et al., and employs a halogen-containing powder or gas, preferably ammonium fluoride (NH4F). Reeves et al. treat the surface of a TBC-coated component at a temperature sufficient to yield halogen ions that are believed to attack the metal oxide bond between the TBC and the bond coat. Reeves et al. note that aluminide bond coats are degraded by this treatment, though the underlying superalloy substrate remains unharmed. While Reeves et al. represent a significant advancement in TBC removal, further improvements are desired, particularly for processes capable of removing TBC from a component surface without damaging the underlying substrate, including any bond coat used to adhere the TBC.
The present invention provides a method of removing a ceramic coating, and particularly zirconia-containing thermal barrier coating (TBC) materials such as yttria-stabilized zirconia (YSZ), that has been either intentionally or unintentionally deposited on the surface of a component. As such, TBC materials with or without a metallic bond coat can be removed by the process of this invention, as bond coats are typically not present on equipment, tooling and maskants used to deposit TBC materials, while bond coats are a preferred constituent on the surfaces of high temperature components in order to tenaciously adhere the TBC to the component, notable examples of which include gas turbine engine components exposed to the hostile thermal environment of the turbine, combustor and augmentor sections of a gas turbine engine. The method is particularly suited for completely removing the TBC without removing or damaging the metallic bond coat, if present, or damaging the underlying substrate material.
The method of this invention generally entails subjecting the TBC to an aqueous solution containing an acid fluoride salt, such as ammonium bifluoride (NH4HF2) or sodium bifluoride (NaHF2), and a corrosion inhibitor. A preferred process for removing the ceramic coating entails immersing the component in the solution while maintained at an elevated temperature, and subjecting the coating to ultrasonic energy. Using the method of this invention, a TBC can be completely removed from the component and any surface holes, such as cooling holes often present in the airfoil surfaces of gas turbine engine components, and with essentially no degradation of a bond coat (if present) or substrate beneath the TBC. Therefore, the method of this invention can be used repetitively without eventually destroying the component or equipment from which the TBC is removed.
In view of the above, the present invention is particular suitable for removing TBC from gas turbine engine components and the thick TBC that accumulates on equipment, tools and maskants used to apply TBC on such components. This invention also allows the deposition of a new TBC on components intended to be thermally insulated with TBC without necessitating refurbishment or replacement of the bond coat and without depositing additional ceramic in any surface holes, e.g., cooling holes. If the component was previously in service, such that the bond coat has been partially depleted as a result of oxidation, the bond coat can be refurbished before replacing the TBC. A significant advantage of this invention is therefore the reduced labor, equipment and processing costs required to refurbish components insulated with TBC and to remove unwanted TBC from the equipment, tooling and maskants used to deposit TBC on such components. In addition, the service life of a component can also be extended by avoiding replacement of its entire TBC system, since removal of a bond coat results in loss of wall thickness, particularly if the bond coat is a diffusion aluminide that inherently shares a significant diffusion zone with the component substrate.
Another advantage of the present invention is that prior art techniques for removing TBC have typically been unable to remove TBC from the cooling holes of air-cooled components, or have caused excessive damage to the bond coat in the process of removing the TBC. With the ability to completely remove TBC from the cooling holes of an air-cooled component without damaging the underlying bond coat, the performance of the component is improved by the ability to restore the TBC to its original thickness, thereby reestablishing the desired film cooling effect at the component surface.
Other objects and advantages of this invention will be better appreciated from the following detailed description.